1. Field of the Invention
The present invention relates generally to a lens system of an image sensor, and more particularly, to brightness correction for a lens in such a lens system.
2. Background of the Invention
A lens generally has curved surfaces such that the curvature of the lens causes light to be concentrated toward a focus center. Thus, light passing through the lens is not spread uniformly. Accordingly, correction for nonuniformity of brightness of an image passed through the lens is necessary. Such a correction is referred to as lens correction or shade correction.
FIG. 1A illustrates the non-uniformity of the brightness of an image transmitted through a lens. FIG. 1B graphically plots the brightness profiles for the R (red), G (green), and B (blue) color components along a horizontal section running through the focus center for the image of FIG. 1A.
FIGS. 1A and 1B illustrate that that light is concentrated toward the focus center for the image transmitted through the lens. For example, the peak of the brightness profile in FIG. 1B is the focus center of the lens with the brightness being the lowest toward the edges of the image.
FIG. 2A shows an image regenerated from the image of FIG. 1A using general lens correction. FIG. 2B graphically plots the brightness profiles for the RGB color components along a horizontal section running through the focus center for the image of FIG. 2A. FIGS. 2A and 2B illustrate that light intensity is more uniformly distributed over the image.
Lens correction techniques are generally classified into radial lens correction or grid lens correction. Profile information for color channels are used for lens correction. For example, channel gain profiles denoting 2-dimensional brightness distributions of R, GR, GB, and B color channels are used for lens correction. R denotes a Red color channel, and B denotes a Blue color channel. GR denotes a Green color channel adjacent to a Red color channel in a row, and GB denotes a Green color channel adjacent to a Blue color channel in a row.
FIG. 3A shows the image of FIG. 1A with a grid formed thereon. FIG. 3B exemplarily illustrates a grid lens correction technique for the image of FIG. 3A, according to the prior art. For example in FIG. 3B, a point E is surrounded by grid summit points A, B, C, and D where respective color correction gains are known.
Thus, the brightness at the point E is corrected using the color correction gains at the grid summits A, B, C, and D. For example, a color correction gain E at the point E is represented by Equation 1 below:E=A*(1−x)*(1−y)+B*x*(1−y)+C*y*(1−x)+D*x*y  [Equation 1]In the equation above, A, B, C, and D are color correction gains respectively at the points A, B, C, and D. x and y above represent the distance ratios as shown in FIG. 3B.
Such a grid lens correction technique is accurate, but requires storage of all data for the grid summits. Such storage of the data of all the grid summits may require costly memory capacity.